This project studied the responses of several classes of neurons located in trigeminal nucleus caudalis responding to noxious and innocuous mechanical and thermal stimuli in awake, behaving rhesus monkeys. Monkeys were trained to detect the termination of innocuous heat stimuli and the onset of noxious heat stimuli. Nociceptive neurons often had thermal thresholds below 45 degrees C, but stimulus response functions were positively accelerating in the noxious heat range. The responses of phasic neurons could provide information about the presence of behaviorally significant stimuli. Tonic neurons exhibited complex response properties primarily characterized by tonic excitation or suppression throughout the execution of certain task related behaviors and/or the presentation of certain task related stimuli. The activity of such neurons may be associated with attentional or motor processes utilized during the execution of certain behaviors employed in this task.